With the increased interest in energy conservation, lighting systems that use less energy and are easy and cost-effective to install are becoming more important. This conclusion is apparent when one considers that lighting uses about 30% of the energy consumed in the United States. An effective method of reducing energy consumption that has been found by the lighting industry is to employ electronic ballasts that energize the lamps with high frequency alternating currents, operating at frequencies in the 20 kHz to 100 kHz range. Electrical energy in this frequency range is efficiently generated by use of sine wave inverter circuitry that converts a DC voltage into the high frequency sine wave power that is coupled to the lamps.
An instant start electronic ballast does not employ any filament preheat mechanism to assist in thermionic emission from the lamp electrodes, but relies upon sudden application of a high voltage between the lamp electrodes to ignite the gas discharge within the lamp. Thus, in an instant start ballast, the ballast circuit must provide a high voltage into the open circuit load that the lamps present before they are ignited. However, after ignition the lamp impedance changes to a low value. This low value of impedance becomes even lower with increasing lamp current, a property of gas discharge electrical loads. This lowering of the lamp impedance with current, known as negative resistance, can cause circuit instability unless an impedance known as the ballast impedance is placed in series with the lamp load. The presence of the ballast impedance helps maintain stable operation, and also plays a role in determining the final lamp current.
For a ballast to operate over a wide range of line voltages, a pre-converter stage may be employed that boosts the incoming voltage to a value higher than the peak value of the highest AC voltage that the unit will use. This pre-converter also known as a boost converter typically uses an industry standard integrated circuit to perform this power conversion in a way such that the AC load current follows the incoming AC line voltage. This methodology, known as Power Factor Correction (PFC), greatly improves the electrical power factor of the incoming AC current. In a large lighting installation this is an important feature as it reduces the amount of re-circulating reactive power in the building wiring and the electrical utility transformers and feeders allowing more useful power to be transmitted through the electrical transmission of the utility company. Power factor correction circuits in prior art ballast circuits can have difficulty in properly operating over the wide or universal AC input voltage range. A circuit that shifts a sampled voltage input in a power factor corrector chip to a more linear region of the chip characteristics would improve the overall performance of the ballast.
The output of the PFC boost converter in an electronic ballast is a DC voltage whose value exceeds the peak value of the highest AC voltage within the operating range of the ballast. An inverter stage is used to convert this DC voltage into the high frequency sine wave voltage required to operate the lamps efficiently. An inverter that is frequently used for instant start ballasts is a self-oscillating half bridge circuit that is current fed through an isolation choke. A cause of heating of the transistor switches in this type of inverter is a particular type of shoot-through current that occurs during the brief switching interval when the transistors switch from one being on to the other being on. This heating becomes larger when reduced power is being converted, such as occurs with shorter lamps are used, or when the number of lamps is reduced. Circuitry that reduce the size of the shoot-through current, and reduces circuitry power losses in the inverter would improve the overall performance and the reliability of the ballast.
During conditions of operation when no lamps are connected to the ballast circuit, overheating of the inverter components may occur. To reduce the overheating in this condition and thus improve the overall performance and the reliability of the ballast, power to the inverter is cyclically switched ON and OFF under control of a zero lamp sense circuit. When a new lamp is installed, normal operation is resumed without disconnection and reconnection of the AC power source as is required by some designs. This provides for the ability of lamp changing within a facility without the need of turning lamps ON and OFF.
Examples of such prior art are shown in the examples that follow.
U.S. Pat. No. 5,177,408, granted Jan. 5, 1993, to A. Marques, discloses a startup delay circuit for an electronic ballast for “instant start” type fluorescent lamps of the type having an electronic converter powered by an active electronic preregulator. The converter is a inductive-capacitive parallel resonant, push-pull circuit or any other type of current fed power resonant circuit. The preregulator may be of a boost type—the startup circuit may be either resistor and Zener diode, or resistor, capacitor and Diac network or programmable unijunction transistor circuit connected between the preregulator output and an oscillator enabling input of the converter.
U.S. Pat. No. 5,214,355, granted May 25, 1993, to O. K. Nilssen, discloses an instant start electronic ballast is comprised of a first and second AC output voltage, where the second AC voltage is delayed roughly 90 degrees from the first AC voltage, which results in the voltage across the tank inductor being approximately sinusoidal in shape. A first and a second fluorescent lamp are connected in series with the first and a second ballast capacitor, respectively and the two-lamp capacitor series combination are connected in parallel across the inductor, thereby resulting in a sinusoidal current being provided to the lamps.
U.S. Pat. No. 5,559,405, granted Sep. 24, 1996, to Hubin [0081] otohamiprodjo, discloses a ballast for operating a gas discharge lamp having a voltage boost, a half-wave bridge inverter and a parallel resonant circuit. An inverter control inhibits operation when the power is initially applied to the ballast.
U.S. Pat. No. 5,834,906, granted Nov. 10, 1998, to J. Chou, et al., discloses an electronic ballast for driving a fluorescent lamp which includes an EMI filter and power circuit, a preconditioner coupled to the EMI filter and power circuit, and an inverter circuit coupled to the preconditioner for energizing the fluorescent lamp. The preconditioner includes an active power factor controller and a boost circuit that is controlled by the active power factor controller. The active power factor controller has a reference voltage input to which is applied a reference voltage. At the startup, the inverter applies a time varying signal that is rectified. At least a portion of the rectified signal is fed back to the reference voltage input of the active power factor controller to boost the reference voltage to a level above normal so that the active power factor controller will cause greater current to flow through the boost circuit, causing the boost circuit to generate a DC rail voltage more rapidly, which rail voltage is provided to the inverter circuit to ignite and operate the fluorescent lamp.
All of the above referenced prior art, disclose instant start electronic ballast circuitry for use with fluorescent lamps. However, none of the prior art teach the use parallel connected lamps that operate over a wide applied operating voltage range; lamps that can be disconnected while the AC line voltage source is applied.
What is needed is an energy efficient instant start electronic ballast that is capable of operating a plurality of parallel connected gas discharge lamps that can be operated over a wide range of applied AC voltages and having an automatic restart capability without interruption of the power source. In this regard, the present invention fulfills this need.
It is therefore an object of the present invention to provide an instant start electronic ballast for fluorescent lamps that can be used over a wide or universal range of applied AC line voltages while maintaining adequate power factor correction over the entire range of operating voltages.
It is an additional object of the present invention to provide proper operation of an electronic ballast having a plurality of lamps connected in parallel, so that light is still produced when at least one lamp is connected to the ballast.
It is another object of the present invention to provide efficient operation of an electronic ballast sine wave inverter circuit, even with smaller lighting loads.
It is a still further object of the present invention to provide for reduced power consumption of an electronic ballast when no lamps are connected to the ballast.
It is a final object of the present invention to ensure that the electronic ballast provides for the automatic restart of operation when a lamp is installed, without disconnecting the applied AC power source.